I. Field of the Invention.
The present invention relates generally to electronic measuring systems and more particularly to an electronic weighing system utilizing microwave frequency dielectric resonant load cells to provide highly accurate digital load measurements including temperature compensation.
II. Description of the Prior Art
Various types of transducers have been developed for measuring loads. In the simplest type of prior art mechanical scales, a load is applied to a mechanical load receiving member, such as a calibrated spring assembly and platform, in order to mechanically control a balance mechanism to provide a weight indication. Such systems are used for example in conventional portable beam scales, bench scales, or health scales.
In addition, prior art electronic systems which measure weight to provide an electronic display based on a load applied to a transducer, such as an analog load cell or a digital load cell, such as disclosed in U.S. Pat. Nos. 3,850,023; 3,665,169; 3,603,298; 4,330,837; 4,143,724; 4,722,406; and 4,137,568 are also well known.
Typically, in prior art electronic scales, a continuous voltage is applied across a load cell arranged in a wheatstone bridge. The voltage across the load cell output is measured, from which the magnitude of the load applied to the load cell is determined.
Another conventional type of load cell employed in prior art scales is a capacitive load cell. In the capacitive load cell, the load cell is connected so that its capacitance will change in proportion with the weight on the scale, an oscillator being connected in the circuit so that the change in capacitance is directly proportional to change in frequency of the oscillator. Such prior art systems are exemplified by U.S. Pat. Nos. 4,273,204, 4,951,764, 4,898,254, 4,917,199, 4,712,627, 4,372,405, 3,314,493, and 4,585,082.
Another type of prior art transducer which has been employed is the piezoelectric transducer, such as disclosed by way of example in U.S. Pat. Nos. 4,623,813 and 4,623,030.
Moreover, heavy capacity prior art weighing systems typically employ a plurality of strain gages mounted at the base of a large weighing platform. These scales are often exposed to the natural elements, which include moisture, temperature variations, humidity and lightning. Further, since these scales are generally expensive and, therefore, used for long periods of time, it would be highly preferable to have them utilize, if possible, load transducers which would not degrade rapidly over time. Furthermore, the accuracy of such scales may be affected by other factors such as electrical noise interference which arises due to the large distance between the strain gauges and the load determining instrumentation.
It would, therefore, be desirable to be able to overcome these problems present in the prior art by providing a load measuring transducer which is less vulnerable to electrical and environmental distortions, which is cost effective, and which is suitable for large capacity weighing systems.